1. Field of Invention
This invention relates to a DC power supply for feeding back a signal which stabilizes an output by using a transformer to insulate a stabilizing signal. More particularly, this invention relates to a DC/DC converter in which the excitation energy of an insulating transformer is obtained from a switching output in a secondary winding of a main transformer. The excitation energy is supplied as a stabilizing signal that is related to the output voltage to a primary coil of the insulating transformer. The stabilizing signal is used for activating a switching control circuit after the excitation energy has been removed by a secondary winding of the insulating transformer.
Furthermore, this invention relates to a DC/DC converter which is provided with means for protecting a power supply by detecting defects such as an overvoltage, overcurrent, or low voltage in a load, and for supplying detected information to a switching control circuit.
2. Discussion of the Prior Art.
U.S. Pat. No. 4,680,688 discloses a multi-output regulated power supply that mainly stabilizes a plurality of output voltages by use of a single feedback path. U.S. Pat. No. 4,685,039 discloses a device for obtaining large output and capacity, wherein the primary winding is so constituted that two one-transistor type ON/ON type converters are connected in series and diodes are connected in a crossing manner, while the secondary winding is so constituted that the two one-transistor type ON/ON converters are connected in parallel.
A device of the latter type has been developed for use as a power supply for a system. It is able to supply a variety of voltages, such as 5, 12, and 28 volts, by means of a single power supply, and is characterized in that the required level of power supply is provided although the volume of the device is small. It is preferable that the power supply for the system satisfies various safety standards, such as required by Underwriters Laboratory standards. Thus, it is necessary for the primary winding and the secondary winding to be completely insulated from each other, which necessitates application without exception of a feedback circuit for stabilizing output.
A power supply which is intended to satisfy such safety standards is exemplified in U.S. Pat. No. 4,215,392 which discloses an inverter power supply which is characterized in that the main path thereof is insulated by a main transformer, and a feedback circuit for stabilization is insulated by a photocoupler. However, the use of the photocoupler causes the following problems.
1. In order to meet safety standards, a certain distance is needed between the primary winding and the secondary winding, which deteriorates the converting ratio (CTR), so that the size of the device becomes large.
2. Since differences in CTR between individual devices, changes caused by temperature, and deterioration caused by time and radiation are too large, the device must be designed to include large tolerances.
3. If a high switching frequency is inputted as it is, response cannot follow the high switching frequency.
Therefore, it has been considered that an insulating transformer could be provided for the feedback circuit. However, if this design is used, the output voltage changes in a similar manner to that of the DC voltage, so that excitation energy must be independently supplied. In that case, a transformed for excitation energy must be provided, causing the structure to become complex, costs to become high, and the size of the device to become large.
Furthermore, it is preferable that the power supply for the system be extremely reliable so that maintenance intervals are lengthened. Thus, if it is considered likely that a defect (for example, a short circuit or a misconnection with another power supply) will occur in the load because of an overvoltage, overcurrent or low voltage, countermeasures for protecting the power supply from damage, must be undertaken.